The kind of meters we used to install 50 years ago would turn backwards if electricity flowed backwards. So if you spent a week with the meter connected normal

The kind of meters we used to install 50 years ago would turn backwards if electricity flowed backwards. So if you spent a week with the meter connected normally, then you swapped the input and output cables around for a week, the meter would be back at zero. Free electricity! They used anti-tamper seals to make it more detectable, but there are ways around that sort of thing.